Computers are widely used to monitor a multiplicity of processes occurring in manufacturing or processing facilities. In order to accomplish this task, sensors are provided to monitor the state and/or condition of particular manufacturing and/or processing variables, and these signals are transmitted to a multiplexer which acts as an interface with the computer system. The multiplexer samples the incoming signals and subsequently transfers the samples to the computer system. The prevailing practice has been to sample these signals periodically at fixed time intervals and to transmit all of the samples to the computer system. With this technique, which is known as "polling", the sampling period must be at least twice the highest frequency present in the process signals and typically it is significantly greater than twice the highest frequency. Because of this requirement for the sampling rate and inasmuch as all samples are transmitted to the computer system and/or a display device, this approach has an inherent problem in that the volume of samples transmitted is enormous which creates an inherent time delay in the transmission of same, thus increasing system response time i.e., the time period required between the occurrence of an event in the plant or facility and the completion of the associated data transmission to a computer system and/or a display device. Such an increase in system response time can result in an unsafe condition within the plant or facility for an inordinate period of time before being detected. In order to minimize this problem, the sampling rate can be decreased, however, such an approach increases the probability that a rapidly changing process variable might not be sampled. Thus, up to the present, the desirability of a fast system response time could not be achieved at a high sampling rate.
Because of the foregoing, it has become desirable to develop a system which has a high sampling rate to preserve system accuracy and in which response time is minimized.